Cathode ray apparatus



Oct. 24, 1939. BOWMAN-MANIFOLD 2,176,973

CATHODE RAY APPARATUS M. BOWMAN-MANIFOLD 2,176,973

CATHODE RAY APPARATUS ilecl June 1, 1935 2 Sheets-Sheet 2 Patented Oct.24, 1939 UNITED STATES RATENT OFFHQE signor to Electric &

Musical Industries Limited, Hayes, Middlesex, England, a company ofGreat Britain Application June 1, 1935, Serial No. 24,473 In GreatBritain July 4, 1934 11 Claims.

The present invention relates to electrical circuits for the deflectionof cathode ray beams.

In certain forms of cathode ray tube, the oathode ray is required toscan a rectangular area on a screen disposed at an acute angle to themean direction of the cathode ray which will be referred to as the axisof the ray. An example of this is a cathode ray tube used for televisiontransmission in which the screen is of the mosaic type and in which theimage to be transmitted is formed upon the same side of the screen asthat upon which the ray impinges. If the deflection is efiected in thenormal manner using electrical oscillations of rectilinear sawtooth waveform and constant amplitude for the deflection in both coordinates, thearea scanned upon the screen will be of keystone shape and the stripsscanned will be more widely spaced in the neighbourhood of the edge ofthe screen further from the source of the cathode ray, or electron gun,than in the neighbourhood of the opposite edge.

It is an object of the present invention to provide means for overcomingthis diificulty.

According to the present invention, there is provided a circuitarrangement comprising a cathode ray tube having an electron gun, ascreen inclined at an acute angle to the ray axis and a deflectingcircuit provided with means for generating electrical oscillations ofsaw-tooth wave form of two different frequencies, the first of saidoscillations serving to deflect the ray in a direction parallel to aplane containing said axis and the normal to the screen at the point ofintersection of said axis with the screen and the second of saidoscillations serving to deflect the ray in a direction perpendicular tosaid plane, wherein the generating means are such that, in the first ofsaid oscillations, the rate of change of voltage or current in saiddeflecting circuit is greater when the ray is on the part of the screennearer the electron gun than when the ray is on a part of the screenfurther from the electron gun and such that the second of saidoscillations has a wave form which is a function of the firstoscillation and of a substantially rectilinear saw-tooth wave.

It is usually arranged that the first oscillation is that of lowerfrequency, namely the framing frequency oscillation, the secondoscillation being the strip frequency oscillation. In this case thestrip frequency oscillation has a rectilinear sawtooth wave form and itsamplitude changes as a function of the frame frequency wave form.

The invention will be described with reference to the accompanyingdrawings, in which Fig, 1 is an explanatory circuit diagram, Figs. 2 to'7 are diagrams illustrating the invention, Fig. 8 shows a circuitdiagram of one embodiment of the present invention, Fig. 9 shows amodification of a part of the circuit diagram of Fig. 8, and Fig. 10shows one circuit arrangement whereby the output of the circuit of Fig.8 is employed for deflecting the beam in a cathode ray tube.

Referring to Fig. 1, there is shown a well known arrangement forgenerating a saw-tooth potential wave, the frequency of the oscillationgenerated being controlled by unidirectional pulses from a sourceindicated by l, the duration of each of these pulses being very shortcompared with the intervals between successive pulses. The anode-cathodepath of the valve 2 is arranged to be normally insulating and acondenser 3 is charged at a substantially uniform rate through aresistance 4, the end of the resistance remote from the anode beingmaintained at a fixed positive potential E relatively to the cathode ofthe valve. When a pulse arrives at the grid of the valve it makes thegrid more positive and the anode-cathode path of the valve 2 conductsand discharges the condenser 3, thus completing the cycle. The timeconstant of the resistance 5 and the condenser 3 is usually made largecompared with the time period of the oscillations to be generated. Theoscillations generated in this way will be of substantially rectilinearsaw-tooth wave form and constant amplitude and one generator of thiskind may be used for the strip frequency and another for the framefrequency.

It will be assumed for convenience that the cathode ray tube has itsmosaic screen disposed so that the plane containing the ray axis and thenormal to the screen (at the point of intersection of the ray axis withthe screen) is vertical. Thus the diagram of Fig. 2 may be taken as aview in elevation, the screen being represented by reference 5, thesource of cathode rays by reference 6 and the ray axis by 0. If normalmeans (electromagnetic or electrostatic) be used to deflect the ray insuch a way that the strips scanned are substantially horizontal, thespacing between adjacent strips will not be uniform. This is illustrateddiagrammatically in Fig. 2 where for simplicity the screen is shown asif it were scanned in only four traverses of the ray, the strips beingnearly perpendicular to the plane of the paper. It will be seen that thestrips are closer together in the neighbourhood of the edge 1 of thescreen nearer the source 6 than towards the opposite edge 8.

Furthermore the ray instead of sweeping out a rectangular patch such asthat shown in dotted lines in Fig. 3 will sweep out the keystone shapedarea shown in full lines. It is of course desired that the ray shouldsweep out a rectangular area and that the strips should be uniformlyspaced as shown in Fig. 4. The time of scanning each strip is of coursethe same and hence so far as the framing deflection (that in thedirection from 1 to 8) is concerned, the angular deflection of the raynear edge 1 is required to be greater than that produced by the normaldeflecting means and the angular deflection near edge 8 is required tobe less than that produced by the normal deflecting means. In the caseof the strip frequency deflection it will be seen that the amplitude isrequired to decrease as the ray moves from the neighbourhood of edge 1towards edge 8.

The desired deflection in the framing dimension can be obtained with atube having normal deflecting means by giving the framing oscillation awave form of the kind shown in Fig. 5. Here the deflecting voltage EF(or current if electromagnetic means are used) increases more rapidlyfor small values of time t, where the ray is in the neighbourhood of theedge 1, than for larger values of t. The curvature of the wave form isarranged to be such as to give uniform spacing of the strips asindicated in Fig. 4.

Similarly the ray can be caused to sweep out a rectangular area on thescreen by applying to strip deflecting means of normal type a voltage(or current) wave of the form indicated by the dotted line 9 in Fig. 6.Here the deflecting voltage Es is plotted against time t. It will beseen that the frequency of the oscillation in Fig. 6 is constant andthat the amplitude decreases proceeding from near edge 1 towards edge 3.The envelope of this saw-tooth wave 9 is constituted by curves l3 whichare functions of the curve of Fig. 5. The effect of'using deflectingoscillations of the form shown in Figs. 5 and 6 is to scan in a planenormal to the axis (such as plane I U in Fig. 2) along a track somewhatas indicated in Fig. '7, but a rectangular area with uniform stripspacing on the screen 5.

A generator of the kind shown in Fig. 1 can be caused to generate a waveform such as is shown in Fig. 5 by a suitable choice of the values ofresistance 4 and condenser 3, the product of the resistance and capacitybeing made smaller than where a substantially straight line wave form isdesired.

The wave form 9 of Fig. 6 can be obtained as follows: If the potential Ein Fig. 1 be caused to change as a function of EF, and if it is arrangedthat the condenser 3 is substantially fully discharged each time thevalve 2 becomes conducting, the potential of the condenser 3 at theinstant of discharge, represented by curve H of Fig. 6, will beproportional to E and may be represented by K-]CEF, where K and k areconstants,

Thus the wave generated will be of the formshown in full lines at l2 inFig. 6. By subtracting from curve I 2 the curve l3, which is a functionof curve II, the curve 9 can be obtained.

One circuit arrangement in which these results can be obtained is shownin Fig. 8. A source I of frame frequency impulses is associated with avalve l5, condenser l6 and resistance 1'! in the same way as theelements 8, 2, 3 and 4 are associated in Fig. 1, a source 58 beingprovided to supply the potential E. The time constant of the condenserI6 and resistance I! is chosen so that the wave form of the oscillationgenerated is of the kind shown in Fig. 5. This oscillation is amplifiedby a valve l8 and fed to terminals 19 which are connected to inputterminals IQ of Fig. 10. In the circuit of Fig. 10, oscillations fed toterminals l9 are amplified by valve 54 and fed to deflecting coils 46 ofa cathode ray tube 59. The tube 59 has an electron gun 60 and a mosaicscreen 6|, the screen 6| being inclined at an acute angle to the meandirection of the electron beam from the gun 60. The axis of coils 46 ishorizontal and current passed therethrough serves to deflect the beam ina vertical direction. A suitable circuit 62 may be provided betweenterminals l9 and valve 54 whereby the potentials at terminals l9 aredistorted so that a current of the wave form shown in Fig. 5 can bepassed through the deflecting coils 46.

Referring again to Fig. 8, a source 20 of strip frequency pulses isassociated with a saw-tooth generator, also of the kind shown in Fig. 1,comprising valve Zl, anode resistances 22, 23, condenser 24 and a sourceof potential 63. This valve 2| is coupled to a valve 25 having aresistance 26 connected between its cathode and earth. The cathode ofvalve 25 is connected through a resistance 21 and a condenser 28 to thecontrol grid of a pentode 29 the anode 64 of which is connected throughthe primary winding of transformer 23 of Fig. 10 to a-source ofpotential 66, terminals 38 of Fig. 8 being connected to terminals 38 ofFig. 10. The secondary windings 44 of transformer 43 are connected todeflecting coils 61 of cathode ray tube 53. Deflecting coils 61 serve todeflect the beam in a horizontal direction. The control grid of thepentode 29 is also connected through condenser 28 and a resistance 3| toa tapping point, which may be variable, upon a resistance 32 arrangedbetween the cathode of valve I8 and earth. In parallel with theresistance 32 are arranged a condenser 33 and a resistance 34 in seriesand a tapping point on resistance 34, which may also be variable, isconnected to the control grid of a valve 35, the anode of which isconnected to the junction point of resistances 22 and 23. Furthersources of potential 68, 69, 10 are provided to supply positivepotentials to the anodes of valves i8, 25 and 35. The negative poles ofsources 58, 63, 66, 68, 59 and 10 are earthed.

The potential E at the upper end of resistance 22 will thus have aconstant component due to the voltage source 63 and a varying componentproportional to the frame frequency wave form. The output of the valve2| will be of the form shown at l2 in Fig. 6. The inclination of thecurve I I can be altered by adjusting the tapping point on resistance34. The subtraction from this wave form !2 of a quantity proportional tothat defined by curve H is obtained by mixing in resistances 21 and 3!the potential drop across resistance 26 with the potential drop across apart of resistance 32 and feeding the resultant, which may be adjustedto be of the form shown at 9 in Fig. 6, to the pentode 29. Theresistances 21 and 3! are of large value compared with resistances 26and 32.

The wave form shown in Fig. 5 may be adjusted without substantialalteration in the amplitude with the aid of a condenser of large valueshown in dotted lines connected between a variable tapping point onresistance l1 and earth. If the condenser 36 is of sufficiently largevalue, the terminal thereof remote from earth remains at a substantiallyconstant potential and the effective value of the resistance I! is thatportion thereof between the anode of valve l and the tapping .point.

The amplitudes of the frame and strip frequency oscillations can bevaried by varying the potentials applied to the upper ends ofresistances II and 23 respectively.

In the circuit of Fig. 8 it may be diificult or impossible to avoidappreciable loss, in the coupling between valves 2! and 25, of lowfrequency components of the signals which are fed from valve 2| to valve25. This loss causes distortion of the frame frequency component of thesignals, that is, of the envelope l! of the saw-tooth wave l2. In orderto eliminate or reduce this distortion the modification of Fig. 8 shownin Fig. 9 may be employed.

Referring to Fig. 9 a condenser 31 is connected in series with condenser24 on the earth side thereof. The common point of condensers 24 and 3!is connected to the control grid of valve 25 and through a grid leak 32to earth. Resistance 26 in the cathode circuit of valve 25 is shunted bya decoupling condenser 39 and the end of resistance 2'! remote fromresistance 3! is now connected to the anode of valve 25. The tappingpoint on resistance 32 is connected through a coupling condenser 46 tothe grid of a triode valve 4|, a grid leak 42 being provided between thegrid and earth. The anode of valve 4! is connected to the end ofresistance 3! remote from resistance 21.

It is preferably arranged that condenser 3? has a capacity of about fivetimes that of condenser 2-; and that grid leak 38 has a suitable veryhigh value. The loss of low frequency components of the signals in thecoupling between valves 2| and 25 is then governed by the time constantof condenser 31 and grid leak 38.

In the input circuit of valve 4! it is arranged that the time constantof condenser 45 and grid leak 42 is substantially equal to that ofcondenser 31 and grid leak 38 in the input circuit of valve 25. Thepercentage loss of low frequency components is therefore substantiallyequal in these two circuits. Curve [3 (Fig. 6) and the envelope I l ofcurve l2 therefore suffer substantially equal distortion and when thesecurves are subtracted by combining the outputs of valves 25 and 4! inresistances 21 and 3! a substantially undistorted resultant of the formof curve 9 is obtained.

In a preferred arrangement the saw-tooth generators operate at lowamplitude and additional stages of amplification may then be pro videdbetween terminals I9 and I9 and between terminals 30 and 3E. Distortionmay be avoided and the wave form may be improved in these amplifiers bythe use of anti-regenerative feedback.

In cathode ray tubes of the type described above it is often desirableto provide means whereby the rectangular area scanned upon the screenmay be moved as a whole in directions parallel to its pairs of parallelsides. Where electromagnetic deflecting means are employed this resultmay be obtained by superimposing an adjustable D. C. component on thedeflecting coil currents.

Where the output stage of a synchronising signal generator is providedwith an output transformer the circuit of the upper part of Fig. may beemployed. The output transformer 43 has a secondary winding comprisingtwo similar coils 44, the outer ends of which are connected through atransmission line 45 tothe deflecting coils 61 of cathode ray tube 59.This tube 59 may be arranged at a point distant from the generators. Apotentiometer 41 is supplied with current from a source 48 throughresistances 49, 50. The centre point 5| and a variable tapping point 52on potentiometer M are connected between the inner ends of thedeflecting coils 53 of a local monitor tube 72 having a fluorescentscreen 13 to enable the area traced out by the ray from the electron gun74 to be observed. The outer ends of coils 53 are connected tothe innerends of coils 44. It will be seen that, with this arrangement, thepotentiometer ll and the deflecting coils 53 of the local monitor 52 aresymmetrically connected at the centre of the secondary winding oftransformer 43. By adjusting the position of tapping point 52 onpctentiometer 37 the magnitude of the direct current through thedeflecting coils 46 may be varied to adjust the position of the rectanular area scanned upon the screen.

The monitor tube 72 is. provided with a second pair of deflecting coils'15 which serve to deflect the beam in a vertical direction. These coilsare connected in series with the deflecting coils 46 of tube 59. Thedeflecting coils 46 and 53 are connected at one end tothe anode of theoutput valve 54 and at the other end to a variable tapping point 55 on apotentiometer 56. One end of potentiometer 56 is earthed, the other endbeing connected to a suitable point in an anode current source 76, thenegative terminal of which is earthed. A condenser 51 is connectedbetween tapping point 55 and earth. Variation of tapping point 55 onpotentiometer 55 changes the magnitude of the direct current through thedeflecting coils 36 and hence moves the area scanned upon screens 6| and13.

I claim: 1. The circuit arrangement for varying keystone and crowdingdistortions comprising means to develop nontriangular wave energy, meansto develop triangular wave energy, means to subtract a portion of thedeveloped non-triangular energy from the developed triangular energy,and means to supply the difference of the two energies to magneticcoils.

2. A circuit arrangement for varying keystone and crowding distortionscomprising means to develop non-triangular wave energy of predeterminedfrequency, means to develop triangular wave energy of a predeterminedhigher frequency bearing a multiple relation to the frequency of thenon-triangular energy, means to subtract a portion of the developednon-triangular energy from the developed triangular energy, and magneticmeans to control a stream of electrons in accordance with the differenceof the two developed energies.

3. A circuit arrangement for varying keystone and crowding distortionscomprising a cathode ray tube having an electron gun for projecting abeam of electrons, magnetic coils for deflecting the beam of electrons,means to develop nontriangular wave energy of predetermined frequency,means to develop triangular wave energy of predetermined multiplefrequency of the nontriangular frequency, means to subtract'a portion ofthe developed non-triangular energy from the developed triangularenergy, and means to supply energy representative of the difference ofthe two energies to the magnetic coils.

4. A circuit arrangement [for varying keystone and crowding distortionscomprising a cathode ray tube having an electron gun for projecting abeam of electrons, a screen inclined at an acute angle to the meandirection of said beam for receiving said electrons, a first deflectingmeans for deflecting said beam, from said mean direction, in a directionparallel to a plane containing said mean direction and normal to saidscreen, a second deflecting means for deflecting said beam in adirection perpendicular to said plane, a first oscillation generator forgenerating a first scanning oscillation of non-triangular wave form, asecond oscillation generator for generating a second scanningoscillation of triangular wave form, the frequency of said firstoscillation being less than the frequency of said second oscillation,said second oscillation generator comprising a condenser, a resistance,a source of potential difference for charging said condenser throughsaid resistance, means for periodically discharging said condenser and acoupling between said source and said first oscillation generator forcontrolling the effective charging potential of said source inaccordance with said first scanning oscillation, means for feeding tosaid first deflecting means an oscillation from said first generator,means for feeding to said second deflecting means an oscillation fromsaid second generator.

5. A circuit arrangement for varying keystone and crowding distortionscomprising a cathode ray tube having an electron gun for projecting abeam of electrons, a screen inclined at an acute angle to the meandirection of said beam for receiving said electrons, a first deflectingmeans for deflecting said beam, from said mean direction, in a directionparallel to a plane containing said mean direction and normal to saidscreen, a second deflecting means for deflecting said beam in adirection perpendicular to said plane, a first oscillation generator forgenerating a first scanning oscillation of nontriangular wave form, asecond oscillation generator for generating a second scanningoscillation of triangular wave form, the frequency of said firstoscillation being less than the frequency of said second oscillation,said second oscillation generator comprising a condenser, a resistance,a source of potential difference for charging said condenser throughsaid resistance and means for periodically discharging said condenserand a coupling between said source and said first oscillation generatorfor controlling the effective charging potential of said source inaccordance with said first scanning oscillation, means for feeding tosaid first deflecting means an oscillation from said first generator,combining means for combining an oscillation from said second generatorwith said first scanning oscillation and means for feeding the combinedoscillation to said second deflecting means.

6. A circuit arrangement for varying keystone and crowding distortionsaccording to claim 5, wherein said combining means comprise twoimpedances, and comprising in addition means for feeding to saidimpedances oscillations from both said generators and means for derivingfrom said impedances a combined oscillation, dependent upon thedifference between these two oscillations.

7 A circuit arrangement for varying keystone and crowding distortionsaccording to claim 5, comprising in addition a first coupling circuitcoupling said first generator to said combining means, a second couplingcircuit coupling said second generator to said combining means, saidfirst coupling circuit being associated with a first frequency selectivecircuit for attenuating low frequencies with respect to highfrequencies, said second coupling circuit being associated with a secondfrequency selective circuit for attenuating low frequencies with respectto high frequencies wherein the attenuation of low frequencies withrespect to high frequencies in said first coupling circuit issubstantially equal to the attenuation of low frequencies with respectto high frequencies in said second coupling circuit. 7 r V V 8. Acircuit arrangement for varying keystone and crowding distortions asclaimed in claim 5 and comprising in addition an output transformerassociated with said combining means and having two secondary windings,a monitoring cathode ray tube having an electron gun for projecting abeam of electrons, a fluorescent screen for receiving said lastmentioned electrons and a pair of deflecting coils, and a potentiometerconnected at its ends to a source of direct current and having twotapping points intermediate its ends at least one of said tapping pointsbeing variable, wherein said pairs of deflecting coils and saidsecondary windings are connected in series between said two tappingpoints.

9. A circuit arrangement for varying keystone and crowding distortionsas claimed in claim 5 and comprising in addition an impedance element, apotentiometer having a variable tapping point and a source of directcurrent wherein said combining means and said impedance element areconnected in series across said source of direct current, saidpotentiometer is connected across said source of direct current, andsaid deflecting coils are connected between said tapping point and thejunction of said source of scanning oscillations and said impedanceelement.

10. The method of correcting for keystone crowding distortion in acathode ray tube, wherein an electron beam is used for scanning, whichcomprises the steps of developing one deflecting field, varying thedeflecting field linearly with respect to time, simultaneously reducingsaid linearly increasing field exponentially with respect to the elapsedtime, simultaneously producing a deflecting field at right angles tosaid first deflecting field, linearly increasing said second deflectingfield, and simultaneously modifying said second deflecting field inaccordance with the first of said deflecting fields, and subjecting theelectron beam to the simultaneous action of said deflecting fields.

11. The method of correcting crowding distortion in a cathode ray, tube,comprising the steps of developing a beam of electrons, developing adeflecting field, and simultaneously varying said deflecting fieldincreasing linearly with time and decreasing exponentially with time.

MICHAEL BOWMAN-MANIFOLD.

